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Update (January 2016): After 3 years of use, the batteries featured in the article below started to suffer from problems associated with old batteries (such as inability to hold enough charge and leakage). I am now planning to replace the cells with fresh ones.

Astrophotographers who regularly travel to remote observing sites require a reliable power source to last an overnight imaging session. In this article, I will describe how to construct a DIY field battery — the most essential component of any portable imaging setup. I will also discuss how to calculate the total power requirement of your system in order to determine the recommended battery capacity (ampere-hours) that will provide continuous power that will last overnight (longer than 12 hours).

A DIY field battery to power my laptop, DSLR camera, mount, and dew heaters

We will use a special type of lead-acid battery called ‘deep-discharge battery‘, a type of battery commonly used in rechargeable devices like emergency lights and Uninterruptible Power Supplies (UPS). Deep-discharge batteries are specifically engineered to have thick solid plates, as compared to spongy plates typical of ‘starting batteries’ used in automobiles. A battery with thicker plates lasts longer and tolerates more the rigorous cycle of charging and discharging compared to a battery with thinner (spongy) plates. Unlike with automobile batteries which are designed to deliver high surge currents in a brief span of time, deep-discharge batteries deliver a more steady current at a much longer period —a characteristic that makes it the battery of choice for powering astronomical equipment.

A lead-acid battery rated at 12 volt (V), 9 ampere-hours (Ah)

Deep-discharge lead-acid batteries are rated in terms of voltage (V) and current (Ah). Typical ratings are 12 V 9 Ah. The higher the capacity of the battery, the longer the battery life (e.g., a 12 V 9 Ah lasts longer than a 12 V 7 Ah battery). To provide continuous power that will last overnight (approximately 12 hours), a battery must have enough capacity.

How Much Power is Needed

Power is measured in watts, the rate at which energy is supplied or consumed. This value is usually specified in electronic devices, but if not, you can easily calculate for power (P) by multiplying the required input voltage (V) by the required operating current (A). In symbols, P = VA.

First we calculate how much energy is needed to last an overnight imaging session. We will use my laptop as an example:

An IBM T42 requires a power supply rated at 16 V 4.5 A and a battery pack rated at 14.8 V 6.6 Ah

The values describe that the laptop requires an input voltage of 16 V 4.5 A. To compute for the energy consumption, we refer to the formula P = VA, which translates to 16 V × 4.5 A = 72 watts consumed for every hour (assuming that the device operates at maximum power). Its battery has an output of 14.8 V 6.6 Ah and thus is capable of supplying 14.8 V × 6.6 Ah = 97.68 Wh. This means that if the laptop operates at maximum power, it will only have 97.68 Wh ÷ 72 W = 1.36 hours of battery life. But then we know that a laptop’s energy consumption depends on a number of factors (e.g., application running, screen brightness, peripherals connected, etc.) and does not always operate at maximum power. In my case, my laptop typically lasts for 6 hours on a typical imaging session. It implies that my laptop operates at a much lower power (lower than the maximum theoretical consumption of 72 W computed earlier, and thus consumes less energy) thereby explaining the extended battery life. To determine its energy consumption, we need to compare its battery life at maximum power with its battery life during a typical imaging session, and then calculate how much energy it actually consumes.

We estimate the power rating through indirect proportion: 72 W is to 1.36 hours = P : 6 hours. The power rating would be 72 W × 1.36 hours ÷ 6 hours = 16.32 W. It means that during a typical imaging session, the laptop only consumes approximately 16.32 watts for every hour.

We need to determine the energy consumption of each equipment in our setup, get the total, and then determine the appropriate energy the DIY field battery should have in order to continuously supply power for the desired number of hours (in this case, 12 hours).

You should come up with a list like the one below. To learn how the other values are computed, click here.

This means that in 1 hour, the setup consumes energy of 29.05 watt-hours. Since continuous power is needed for 12 hours, this would translate to 29.05 watt-hours × 12 hours = 348.6 watt-hours for 12 hours.

A typical 12 V 9 Ah deep-discharge battery can supply energy of 12 V × 9 Ah = 108 Wh. This energy output of 108 Wh is far below the required 348.6 Wh. Since our setup requires 29.05 watts for every hour, the 108-Wh of energy coming from battery can only supply power for 108 watt-hour ÷ 29.05 watts = 3.72 hours. The solution would be to consider a battery with a higher capacity, or to connect a number of smaller-capacity batteries in parallel.

In my setup, I have used 4 identical 12 V 9 Ah deep-discharge lead acid batteries connected in parallel. The total current of the system in this case is 9 Ah × 4 batteries = 36 Ah. This results to a field battery with a rating of 12 V 36 Ah, which translates to a total energy of 12 V × 36 Ah = 432 Wh. It should be able to power my imaging setup for 432 watt-hours ÷ 29.05 watts = 14.9 hours.

The procedure on how to construct the DIY battery is described below. For this DIY, you need identical batteries to supply enough power, a plastic housing, some solid copper wires, a battery connector, a switch, a cutter blade, and a soldering iron.

WARNING: I will not be responsible for any damage caused to your equipment. Follow instructions at your own risk!

The DIY Field Battery

Place the batteries inside the plastic housing. I specifically stated the use of a plastic (non-metal) housing to avoid accidental shorting. Use the cutter blade to trim down the sides of the housing and fit the batteries perfectly.

Four lead-acid batteries housed in a plastic toolbox

Connect the batteries in parallel. Attach the battery connector (cigarette lighter socket) observing correct polarity (the central contact point is connected to the positive terminal and the outer contact point is connected to the negative terminal). Insert a switch along the electrical path. Use a thick solid copper wire and then solder carefully all the connection points.

Four 12 V 9 Ah lead-acid batteries connected in parallel to produce a 12 V 36 Ah field battery. Note: If you wish to insert a fuse (for protection), use this diagram instead.

The field battery is now ready for charging.

Charging the Field Battery

The DIY field battery must be charged with an appropriate charger. The charger should match the voltage (12 V) and should deliver enough current to the battery.

A typical example of a 6-A battery charger available in most hardware stores and electronics shops. This particular charger can charge batteries with capacities of up to 60 Ah (see table on the front panel).

To charge the battery, connect the charger to the battery observing correct polarity: positive to positive (red), negative to negative (black). Since the field battery has a capacity of 36 Ah, based on the table, the charging time should last for approximately 6 hours.

For chargers with an ammeter, upon connecting the charger to the battery, the meter indicates the amount of current being sent to the battery at any particular instant. This is very helpful in determining the battery status during charging.

Upon connecting the charger to the battery, the meter abruptly moves then stabilizes in a few minutes (left). During charging, the meter should continuously and very slowly (in a span of hours) indicate a slow decrease in the amount of current sent to the battery until it reaches a point that it stops, indicating that the battery is now fully-charged (right).

Disconnect the charger as soon as the battery is fully-charged. The field battery is now ready to use.

To power various equipment requiring voltage input other than 12 V (e.g., laptop requiring 16 V, a camera requiring 7.4 V), you may just use a DC to DC converter. (Note that while power inverters would also work, that is, changing 12 V DC to 110 V/220 V AC then plugging in the appropriate AC to DC adapter/converter to obtain the required voltage, I would not recommend its use. It is rather inefficient since a lot of electrical energy is wasted as heat. Inverters must only be used if absolutely necessary.)

I have been using this field battery since November 2011 and have not had problems powering my portable imaging setup. At my current load, the field battery lasts for approximately 15 hours, more than enough power for an overnight imaging session.

I just wanted to let you know that I followed your DIY field battery. The instructions you gave were perfect. I also added some red lighting to help me out in the dark. I just wanted to say thank you. Wish I could post a picture.

I was wondering if you could help me out… I talked to some folks at another astronomy community and they’re saying that my suggested field battery wont be able to supply enough power to my laptop.

I was planning to build a 12v/18ah battery to supply my laptop that’s rated at 19v/3.42ah. I know also for a fact that the laptop uses less than that because the battery is rated 10.8v/4800mah and this lasts me for a good 4 hours.

(1) Since your laptop is rated at 19V at 3.42 A, it means it consumes 19V X 3.42 A = 64.98 W of power. We will use this value in number 4.
(2) We also know that your laptop has a battery pack rated 10.8V at 4.8 Ah, which means it can deliver 10.8V X 4.8Ah = 51.84 Watt-hours of power, and according to you, it can power your laptop for approximately 4 hours. It implies that during your usual imaging session, the laptop only consumes 51.84 Watt-hours / 4 hours = 12.96 Watts for every hour. We will use this value in item number 5.
(3) The field battery you are planning to build has a total available power of 12 V X 18 Ah = 216 Watt-hours.
(4) At maximum power consumption, this amount of available power will only last for about 216 Watt-hours / 64.98 W = 3.32 hours.
(5) During your usual imaging session, the same amount of available power will last for about 216 Watt-hours / 12.96 W = 16.66 hours.

This is also the same value that you will get if you try to do some ratio and proportion.

(6) The laptop’s battery pack has a total of 51.84 Watt-hours of power, good for 4 hours (based on your reported usage; see item 2).
(7) The battery you are planning to build has a total of 216 watt-hours (see item 3).
(8) Through ratio and proportion: if 51.84 Watt-hours should last for 4 hours; 216 Watt-hours should last for a certain number of hours. Solving for the unknown will yield 16.66 hours. It also means that the field battery you are planning to build is roughly 4 times more powerful, and should last 4 times as long :)

If you only intend to power your laptop, and nothing else (no cameras, no mount, no heaters, etc.), it should be able to last 3.32 hours (running at maximum consumption) up to 16.66 hours (during your usual imaging session). Hope this helps!

Hello and thank you for your great DIY! I have constructed the field battery and have used it on several occassions using the same batteries and components. I checked the math and it all works out. I am using the Celestron AVX, Macbook (Black 2008), and a Dew Heater. I should be able to power my setup for hours, however, my Macbook will charge for an hour and then stop. Battery charger indicates that it is connected but not charging.

I am not sure if maybe the power flow drops (~12V) with everything connected to the same battery and thus cannot consistently power the laptop and thus no charge. Have you experienced anything like this?

Victor I have heard of a laptop with this problem but it was caused by an aftermarket charger that had a missing center pin that sends voltage or some data to the laptop to let it know when to stop. In your situation since u fear a low voltage what u need to do to test your theory is first try charging in your car and see if it stops after one hour. If it does the charger is at fault. If not u need to buy a 12volt splitter. Connect a 12volt voltmeter (automotive type from amazon.com $15) and notice the voltage when u start charging and when u stop. It could even be as simple as u needing to test using a better 12 charger or a USB port that provides more amps thatn what u r using now. So my best guess is u r not using a adequate 12 charger even tho it works fine on ac power. Or u could be using an inverter. Try one specific to 12 volt power etc. Test several chargers.

Can you provide the specs of your DIY battery and charger (voltage/current, etc)? I suspect that not enough power reaches the laptop. For the laptop to indicate a ‘charging’ status, it requires a certain amount of power, which I believe is adequately supplied within the first hour of use, and then drops eventually. It is possible that there is a problem with either (1) the battery itself, not supplying enough power, (2) the converter/inverter, also not supplying enough power or probably supplying too low voltage or current. We need to determine first where the problem lies. Maybe you can measure the actual output of the laptop charger when connected to AC, and then compare it with the output when using the field battery.

Connect the charger/inverter/converter to a car’s cigarette plug as Bruce suggested. If still the same thing happens (power dropping after an hour), then probably, (1) a charger/inverter/converter problem, or (2) car plug also not supplying enough power. If no problem occurs, then it is most likely an issue with the battery. Alternatively, you can test several chargers as well, to determine if you have a faulty charger. Let us know how it goes :)

I’m a new reader in your site. Having done some simple astro-photography way back in the mid 80’s while I was living and working in the USA. All that I had with me was a used Minolta SRT 101 SLR camera (with mirror lock up feature) some Kodak 3200 ASA Film fresh from the refrigerator, a cable release, a sturdy Bogen Tripod, my watch. I timed and bracketed my exposures and jotted all these down in an old notebook. No laptops yet then!

Viewing the magnificent and inspiring astro-photos of yours got me interested to re-kindle this hobby again but now, I’m using a Nikon D3100 DSLR camera and I just got myself a Celestron C70 spotting scope, T-Adapter and just waiting for the Camera Adapter to arrive in stock.

I live in San Juan and I’m hoping to be able to catch a glimpse of a clear night sky and do some astro-photography once more on the roof deck of my condo unit.

Built this battery about 4 years ago. Used it only a few times and it worked great. Tried to use it last night but it only worked for a few minutes. I thought it was the charge so i put it on the charger. It only charges up to 75%. Are my battries dying?

It is normal for batteries to experience reduced charging capacity as they age. This is true even for old batteries that have never been used. After 2 years, you’ll start to notice this degradation, which is most likely the case with your battery (or any other battery for that matter).

It should still power my laptop for a while, right? I went to use it the other night and it ran my laptop for only a few minutes. It had worked for hours in thr past. I was wordering if ot might be because of the charge or if there’s a problem with my converter?